Glow worm Space Saver 50 MK11 fault?

Please can anyone help? Boiler keeps tripping out. Pilot light stays on fine. When you turn the 'min-max' switch to mid way, the boiler comes fully alight for a minute or two and then completly trips out. You can get the pilot light on again straight away again but just can't get burner to stay on when demand is on. Any advice would be greatly appreciated.